At present, large quantities of methane are expelled from oil wells and natural gas reserves. Most of these wells are located in remote areas, too distant from cities or chemical processing plants to make transport of the methane practical by pipeline. As a result, most of the well operations treat the methane an expendable by-product, which is either burned or vented directly into the atmosphere. This is very undesirable, since the burning or venting of the methane into the atmosphere leads to atmospheric pollution and increases the "greenhouse effect" on the planet. In addition, the value of the methane as a resource is wasted.
In some instances, methane is converted to a liquid that can be transported via truck or pipeline. The technique which has been practiced for decades utilizes the Fischer/Tropsch process. The methane is converted to "water gas", i.e., CO and H.sub.2, and then catalytically transformed into a mixture of hydrocarbons suitable for use in internal combustion engines. This process, however, is not only considered environmentally undesirable, but it cannot be justified when compared with the cost of processing available crude oil.
Many other methane conversion processes have been proposed, but their lack of practicality, for one reason or another, has thwarted commercialization.
For example, a current benchmark process suggests a three-step technique: (a) steam reformation (b) methanol synthesis, and (c) MTG; i.e., EQU [CH.sub.4 +H.sub.2 O].fwdarw.[CO+H.sub.2 ].fwdarw.[CH.sub.3 OH].fwdarw.[C.sub.n H.sub.m +H.sub.2 O].
In the most favored approach, the oxidative coupling of methane is utilized, mediated by suitable metal catalysts: EQU [2CH.sub.4 +O.sub.2 .fwdarw.C.sub.2 H.sub.6 +H.sub.2 O]
These processes, however, require continuous and careful balancing between homogeneous and heterogeneous reactions. The catalysts have to be periodically regenerated or replaced and are economically unfavorable. Conversion levels are too low. Current wisdom has it that this type of process cannot be optimized beyond its present limits.
Another suggested process, which has been tested only in the laboratory, calls for the direct partial oxidation of methane to methanol. This process is not attractive because it requires precise timing and temperature control, as well as a rapid quench step, all of which make commercialization impractical.
In U.S. Pat. No. 4,199,533 issued to Benson, a process for converting methane to higher molecular weight hydrocarbons by using expensive chlorine gas is described. The chlorine gas is thereby converted to inexpensive hydrochloric acid. Also, the use of chlorine as the catalyst presents serious safety problems.
Processes described in U.S. Pat. No. 4,704,496; 4,727,205; and 4,727,207 utilize the feature of free-radical initiators to facilitate methane conversion under homogeneous (thermal) conditions. While the patented processes suggest operative limits that start at 800.degree. C., and extend to 50 atmospheres of pressure, the experimental data presented in these disclosures do not justify these extended limits. The operative temperature is believed to be viable only above 1,000.degree. C. The use of narrow-bore reaction tubes suggests that the conversions incorporate a substantial portion of surface reactions; therefore, this mitigates against the possibility of upscaling to commercially practicable reactor size. All of the above limitations suggest conversion processes that are not commercially viable.
The prior art also suggests a process of methane conversion utilizing various free radical initiators of questionable efficiency. N.sub.2 O as an initiator does generate copious levels of atoms that induce C-H fission and the subsequent chain propagation necessary for the production of higher molecular weight hydrocarbons. The difficult with this prior art technique lies in the high cost of the N.sub.2 O initiator, which is consumed in the reaction in large quantities.
The present invention teaches an apparatus and method for commercial conversion of methane to higher hydrocarbons that are generally in short supply, e.g., ethylene, propene, and butane. The production of these higher molecular weight hydrocarbons and their commercial value aid in justifying the cost of the conversion process. The inventive conversion technique utilizes small amounts (generally at or less than 1%) of a low-cost initiator; this allows for the commercial viability of the process.
The current invention modeled test at low temperatures in the general range of 800.degree. to 900.degree. C. The conversion is viable over a wide range of methane pressures, generally at pressures at or below 20 atmospheres. The free-radical generator present at low concentrations is augmented by a comparably low level of oxygen. The process is not subject to a runaway oxidation of methane; thus, it achieves conversion without precise control of residence time in the reactor. Residence time in the reactor can range widely between 10 to 1,000 seconds. The reaction is carried out in a large-bore, thermal-flow reactor, which negates the possibility of unwanted surface reactions but enhances the throughput.
Model computations show that the presence of low levels of higher hydrocarbons, such as ethane, propane, n-butane and iso-butane, naturally comixed with methane effluents from gas and oil wells, enhance the conversion efficiency of methane as practiced in this invention. Furthermore, the presence of low levels of hydrogen sulfide, if less than 1%, does not reduce conversion when oxygen is added at a slightly higher level than 1%.